Such composites are used extensively in the building construction sectors, automotive/transport sectors, for chemical resistant applications, and in sports and leisure applications notably boat building. The conversion of the polyester resin and reinforcing fibres to form the required structure, consitutes the moulding process. In Europe, a significant part of the structural composites sector utilises open moulds and spray-up or hand lay-up as the moulding process. Generally, the requisite parts are three-dimensional in shape which require application of resin to vertical mould surfaces. The polyester resins should not be excessively high in viscosity for satisfactory spray/roller application. Conversely, low resin viscosity would result in extensive drainage from mould/part vertical surfaces. A satisfactory solution is reached by modifying the rheology of the liquid resins by incorporation of thixotropic structuring agents.
The thixotropic agent used predominantly in polyester resins is fumed silica. Although fumed silica does confer the required structure-imparting properties, it suffers from a number of limitations. One such disadvantage is due to the fine dust nature of the material, which makes it unpleasant and hazardous to handle. Another drawback is the requirement for high shear, capital intensive dispersion equipment for satisfactory incorporation. Lastly, fumed silica has a tendency to sediment in the resin on storage with consequent loss in structure and thixotropic performance.
The latter problem (sedimentation or separation) is particularly performance-limiting and is exacerbated by the use of low viscosity resins now popular in spray/lay up. This sedimentation effect is often rate-determining in designating shelf life.
Corrective procedures before use of the resin involve redispersion, which may not be practical in bulk storage or 1 cubic metre totes. Use of sedimented resin can result in generation of defective parts and scrap. A number of additives can be used in conjunction with the silica that can slow down the sedimentation, but none of them completely alleviates the problem.
Consequently, improved thixotropic solutions have been sought for many years. It is a reflection of the difficulty of the challenge, that despite the efforts invested, fumed silica remains the predominant thixotropic agent. Other products have come to market which counter the sedimentation problem, but impart drawbacks on other parameters. Such products include clays and organo-modified clays which necessitate a "pre-gel" stage for their incorporation. However, with these alternative materials, colour, air entrapment, gel time stability and mechanical properties may be adversely affected.
Thus, there remains a need for a partial or total replacement for fumed silica for the purpose of imparting thixotropy to resins intended for production of fibre-reinforced composites which does not have the disadvantages of the latter material, does not introduce different, significant disadvantages of its own and does not incur a significant cost penalty. This need has now been met by using an organic amide as the agent for producing thixotropic shear thinning.
It is already known to incorporate amides in coating compositions which contain an unsaturated polyester resin and a co-polymerisable unsaturated monomer such as styrene. In particular, such a coating composition is used for gelcoat compositions. Gelcoats are used as a protective layer, for example on the hulls of boats made from GRP. According to a current product data sheet of Kusumoto Chemicals Ltd, such an amide may be incorporated in gelcoat compositions in a form predispersed in a non-polymerisable organic solvent (xylene) such that the total amount of xylene is present at from 2% to 3% by weight of the composition. However, such a solvent level in a composition for making fibre-reinforced structural composites would result in a very poor quality product. Moreover, this data sheet provides no motivation for the person skilled in the art to use such amides in resins for making fibre-reinforced structural composites.
Another data sheet of Kusumoto Chemical Ltd suggests incorporating a polyamide dispersed in styrene monomer into moulding compounds (i.e. compositions of resins with chopped glass fibre for making press-mouldings) as well as gelcoats and other coatings. The amount of amide proposed is equivalent to a level of from 0.05% to 0.3% by weight in the moulding composition. The compositions of the present invention are especially, although not exclusively, intended for use in fibre-reinforced compositions of the laminated type, generally those produced by hand-lay and spray-up. There is no simple extrapolation from moulding compounds to laminated structural applications for a number of reasons. The chopped fibres in the moulding composition result in a different rheology and moulding compositions contain very high levels of filler, typically calcium carbonate. For laminates, thixotropic shear thinning is essential whereas for moulding, only thickening is required (for ease of handling and to prevent separation of the resin from the filler and chopped glass). Moreover, the maximum amount specified (0.3%) in the aforementioned data sheet makes it totally unsuited to laminate applications.